ABUJA, Nigeria (AP) -- A reclusive former governor from Nigeria's Muslim north was sworn in as president Tuesday, in the first transfer of power between civilian leaders in Africa's most populous country.

Umaru Yar'Adua used his inaugural speech to call for an immediate end of hostilities in the southern oil region, a conflict that has cut output by nearly a third in Africa's biggest petroleum producer. Oil prices fell Tuesday as the main militant group in the region said it was considering the request, reflecting hopes that the inauguration of a new president in Nigeria would contribute to stability in the market.

While Yar'Adua's inauguration marked a milestone in a country struggling to consolidate democracy after decades of military rule, he was elected in an April vote the opposition has denounced as fraudulent and international observers have said was not credible.

Yar'Adua, 56, addressed the voting controversy head on, acknowledging at the start of his speech that the elections were flawed. He promised to set up a panel to examine the electoral system ''with a view to effect reforms.''